This blog is intended as an occasional diary of information to feed back to hoverfly recorders in the UK and elsewhere. Inevitably there will be issues of interest that are in some way relevant to invertebrate ecologists and consequently I intend to use the medium as an opportunity to develop thoughts on pertinent topics.

Monday, 28 March 2016

Yesterday I looked at differing emergence times at various latitudes. The alternative is to look at differences in eastern and western Britain.

Unfortunately I do not think there are quite enough data for northern England and Scotland, but there are plenty of data for 2015 for England and Wales south of the Humber-Mersey line. So I have split this area into four - firstly split at the 200 km level (approximately Thames-Severn) and then again at the 200 km mark - in essence using the junction between ST and SU northwards to the junction between SJ and SK.

The results for 2014 and 2015 are presented below. They are not entirely convincing but I think it is possible to detect slightly earlier emergence in SW England. More analysis is needed!

Figure 1. Phenology of Eristalis pertinax in the east and west Midlands (west includes Wales) in 2015.

Figure 2. Phenology of Eristalis pertinax in south-east and south-wet England in 2015.

Sunday, 27 March 2016

I quite frequently use a sequence of processes to smooth data. In this short sequence I hope to show how the data change as a consequence of the process.

In many cases day-on-day records for individual species are too few or too volatile to be particularly meaningful. This is especially true in early spring and to a lesser extent during weekdays in more productive times of year. The solution is therefore to break data into blocks. In this example, I have used Eristalis pertinax, a very abundant spring species that starts to dominate the data from around the end of March. I have extracted photographic data for the past three years (2013, 2014 and 2015) to provide a bit more context. In each of the three years, the data have gained strength because there are more active recorders and therefore more records from all parts of the country (especially northern England and Scotland.

The data I have used have been split into three zones - based on 200km sections of the OS grid that roughly equate to:

South of a line between the Thames and the Severn

The Midlands between the Thames-Severn line and a further line between the Humber and the Mersey.

North of the Humber-Mersey line.

Occasionally, I add a further division north of the Solway, but on this occasion I have not.

The first stage is to generate data for each successive week (Figure 1). As can be seen, the data vary hugely from week to week and inevitably the numbers of records differ for each zone and for each year. Over the three successive years it is very clear that the volumes of data have increased dramatically, which makes it a bigger challenge to put the successive years into context.

Figure 1. Weekly records for the years 2013 to 2015 for Eristalis pertinax according to three zones of latitude.

From this, I construct a further table in which individual weeks' data are converted into a percentage of the total records for the zone in that year. Again, the results tend to be a bit volatile, but they now equate to one another because the effects of the massively greater numbers of records from the south are put into context (Figure 2).

Figure 2. Proportion of records of Eristalis pertinax for each week according to year and zone.

Finally, I run a three-week centred running mean for the data created as percentage of total records. This finally smooths the results because the combination of each three week period is averaged. This removes the idiosyncrasies of big gains and falls in the data and picks out the overall trend - either rising numbers of records or falling numbers of records (Figure 3).

Figure 3. Three-week centred running mean for records of Eristalis pertinax for each zone in the years 2013 to 2015.

The resulting graphs clearly show how the overall phenology pattern differs according to latitude, and how emergence times also vary from year to year. In this example, the indications are that E. pertinax emerged a little earlier in 2014 than in either 2013 or 2015, with the possible exception of the North in 2013. Unfortunately, the dataset for the North in 2013 was rather sparse and therefore I would treat the result with a little caution. Nevertheless, the data do help to show year-on-year regional and latitudinal variation.

The results also show how emergence times are far more protracted in the Midlands and in Southern England, whilst in the northern zone emergence times are far more compressed (hence the higher peaks in 2014 and 2015).

Application

This sort of analysis is only possible for species where there are substantial blocks of data. Nevertheless, it is clear that photographic data can be used for some forms of monitoring and quite remarkable levels of precision in representing species' phenology.

If the numbers of recorders in more northerly regions improves, then it may be possible to break data into four zones in future, thus providing the opportunity to investigate a much wider range of parameters. There may also be scope to split the country further into east and west sub-units of each zone - something I may try in due course.

Saturday, 26 March 2016

Many members of the UK Hoverflies
Facebook page record everything they see on a daily basis; others are
confined to weekends, but are really very active. The combined
efforts of this increasing band of recorders are showing what
can be done with less well-known groups of invertebrates.

What I particularly like is the fact
that there is no discrimination between records of common or rare
species. All records get logged, and as a result we are starting to
build a more realistic picture of the relative abundance of
hoverflies across the country and at particular times of year. This
approach is essential if we are to truly understand what is happening
to wildlife.

Which dates matter?

It is all very well recording first and
last dates, but what happens in the
intervening days/weeks/months is really crucial. First/last dates are rather serendipitous whereas the median date for all records is far more reliable and meaningful. That means that we need all data and an army of diligent recorders. I suspect that data is pretty
robust within moth recording where lots of people make daily counts.
The same probably holds for butterflies and maybe dragonflies. But, can we say the same for
other invertebrate groups? Probably not.

So, it would be nice to think that the
advent of the UK Hoverflies Facebook group heralds a major forward
step for hoverflies. Feedback I have received from some specialists
is not always positive – the question 'why bother with
common species' has been posed, and it has been argued that 'this does nothing to advance our knowledge of
Britain's fauna'. It remains to be seen whether they are right, but I would like to think there is developing evidence to the contrary.

What can be done with the data?

Two small analyses that I undertook
today illustrate the benefits of mobilising an army of recorders with varying levels of experience. To my mind, enthusiasm matters as much as experience. People with enthusiasm will go out when conditions are less than perfect; conversely, those of us who have yaers of experience will look out of the window, note that there are clouds and it is slightly windy, and settle back to the computer. microscope or whatever we were doing.

The data largely show this. Hitherto, hoverflies have been the preserve
of a relatively small body of recorders. For forty years, about 20
people supplied 50% of the data assembled by the Hoverfly Recording
Sscheme (HRS), very few of whom would have made much effort until the spring had fully arrived. The dynamics have changed, and although we still have
a nucleus of about 25-30 highly productive specialist recorders (above 500
records a year), we have many more whose combined efforts are really
helping to develop a valuable picture (several of whom now contribute many hundreds, or in one case thousands, of records). The new recruits start from a very different perspective and want to get out.To illustrate this, I offer two brief
analyses relating to patterns of spring emergence.

Composition of the early spring fauna

If one starts with
the question 'how does spring 2016 compare with previous years?' We
can break the data into the numbers of records of individual species,
but this does not take account of variations in the numbers of
recorders. So, presenting data as a proportion of all records
received for a given timescale is one option.

Figure 1. Records of hoverfly species in March 2014 to 2016 presented as the proportion of all photographic records covering the period 01-25 March.

Figure 1 illustrates this quite nicely:
it shows how, in the period 1-25 March, relative composition of
records changes from 2014 to 2016. This figure is based solely on
photographic records, which is really useful because relatively few
entomologists are stirring from their winter hibernation. The photographic team, however, is raring to go and every time there is a bit
of sunshine they are out. I seriously doubt one could manage to
secure this sort of picture from data prior to 2013, although it
would be possible later in the year when insect activity increases
and traditional entomologists stir from their torpor!

What is very clear is that the composition of the fauna in different years can vary considerably amongst the more abundant species. As might be expected about a quarter of the total species list makes up the bulk of the records, but within this there are two obvious trends. In 2014 and 2016, Eristalis pertinax is far more prevalent in the data than in 2015. There is an obvious reason when you look at the composition of 2015 data: In 2015, Episyrphus balteatus, Eristalis tenax and Meliscaeva auricollis make up far more of the dataset. These three species are typical of winter months and E. balteatus and E. tenax predominantly over-winter. Thus it seems that 2015 was a later year than either 2014 or 2016 where Eristalis pertinax was much more abundant. E. pertinax, in contrast does not generally over-winter and thus the dominance of this species in the dataset tends to suggest that spring has arrived a little earlier.

And at species level?

We
have known for a long while that species' phenology changes from year to year,
and that in some emergence times have advanced considerably.
These changes are nicely illustrated by Epistrophe eligans, which
is highly responsive to spring temperatures; its spring emergence is now several weeks
earlier than it was 40 years ago. We can now pick this up because it
is easily photographed and recorded. In the past, many recorders
might have logged the first record but all too often I have been told
'I'll give first and last dates but I cannot be bothered with the
rest'. Now we have the whole lot! And it beautifully illustrates
differences in both season and latitude.

Figures 2 and 3 present the data for
Epistrophe eligans in 2014 and 2015 for three zones: south (to
the 200km division across the UK, midlands (200-400km) and north –
above 400km i.e. pretty well above the line between the Humber and
the Mersey. What a difference it shows. Emergence times are separated
both between the years and across the latitudes. To do this, I have
separated the data into weekly blocks from 1st March onwards. The
data were then turned into a weekly percentage of total records,
after which they were turned into a centred three week running mean.
This smoothing process helps to refine the picture and it is pretty
powerful.

Figure 2. Phenology of Epistrophe eligans in 2014 based on photographic records.

Figure 3. Phenology of Epistrophe eligans in 2015 based on photographic records.

These graphs suggest that spring in 2015 came a little earlier than in 2014. It remains to be seen how 2016 fares, but the records of E. eligans to date point to a situation more akin to 2015 than to 2014, albeit potentially marred by bad weather forecast for the coming week.

And the moral of the story?

Commoner species matter! One gets too few
records of rarer species to develop strong pictures of change; but at least in some commoner species one can detect change very nicely. That does not
mean there is no place for recording rarer species or taxonomically
difficult species – it is essential, but it may only ever be the
preserve of a small sub-section of recorders.

And, for that splendid band of photographic recorders? I hope these brief examples show just how important your contributions are. This level of activity starts to elevate the hoverflies from the margins to the mainstream of insect recording.

Saturday, 19 March 2016

In my younger days I spent many
weeks/months/years clearing Sycamore from local sites. At the time the mantra
was that Sycamore was an undesirable and invasive alien. Many of the
allegations against sycamore have some foundation; it is undoubtedly capable of
taking over sites where it can readily gain a foothold. Such sites are often
secondary woodland and decaying urban open space. My local site, Mitcham
Common, had a significant issue with Sycamore: many of the youngest woodlands were
being over-run by young trees. So, we cleared the mature trees (I spent a lot
of time gaining valuable chainsaw skills on these trees) and had our teams
pulling the seedlings. Today, sycamore is nothing like the problem it once was.
But, were we right to take such drastic action?

Some years ago, Ted Green published
a think piece in British Wildlife in
which he described Sycamore as 'Northern Plane'. At the time, I thought 'I
wonder if Ted has struck a more sensible note?' Certainly, if one travels
north, Sycamore is a very important component of our landscape, and does not
appear to be the problem that I had encountered further south. Indeed, I think
we could almost say it is a fundamental part of the ecosystem on northern
England and Scotland. For example, I recall one evening in Perthshire, around 9
pm, when we came across a huge Sycamore in flower that was literally buzzing.
Closer inspection revealed that this tree was the focus of huge numbers of
bumblebees; hence the buzzing sound. Obviously it was important in a local
context and I wonder whether it plays a bigger role than we think as a nectar
source of bumblebees (across the British Isles)?

After many years investigating
Sycamore I have reluctantly concluded that it actually plays quite an important
role in the woodland ecosystem too. True, it is fast-growing, shades out other
trees and perhaps vernal flush species, and does not fit neatly into the
perceived wisdom about woodland ecology. Yet, its wood and bark has many of the
important characteristics demanded by epiphytic lichens, and the rot processes
lead to excellent rot holes for Diptera and to very nice sap runs that support
many of the species that native Elm once did. In parkland situations, old
Sycamores can be immensely important habitat for saproxylic Diptera.

Ancient Sycamore at Burghley Park. The rot hole is of exceptional size and is indicative of the tree's value for Saproxylic invertebrates.

Not only is Sycamore good for
saproxylics, it is also a fantastic tree to monitor when looking for
hoverflies. If I visit a woodland and the ride lacks flowers I am more than
happy to monitor the sunlit leaves of Sycamore, especially in the early spring.
These leaves are ideal for leaf baskers and are a standard place for finding Brachyopa adults. The flowers are also
great for hoverflies and for solitary bees. Meanwhile, the leaves are often
infested with aphids that are favoured by a wide range of hoverfly larvae. Unfortunately,
the numbers of Lepidoptera utilising Sycamore are low, and the biomass they
provide for birds is correspondingly low. Nevertheless, in a controlled
situation Sycamore is not the threat it is perceived to be.

Thinking more broadly, Sycamore
coppice has many strengths: it is fast-growing and produces good biomass; the
timber is relatively light and is readily transported; and the root system
develops like other coppice stools and generates excellent decaying wood. There
is therefore something of a case for thinking about Sycamore as a possible way
of facilitating rapid carbon capture and use in sustainable fuel provision.

Let us not get too carried away by
its strengths. Sycamore can be very invasive, its leaves support a very limited
invertebrate fauna, and it does produce an awful lot of shade and leaf litter.

Thinking in a broader context, I
have the good fortune to live in Stamford and to have the wonderful Burghley
Park on my doorstep. I visit the park almost daily and spend a lot of time
gazing at the wonderful hybrid Limes. These magnificent giants are, again,
hardly native; but they have fantastic saproxylic features. The older examples
are substantially hollow and support big Ganoderma
fruiting bodies. Some also have fantastic sap runs, so beloved by a wide range
of Diptera and Coleoptera; and, again, they can be great for aphids and species
that are predacious upon aphids. In some ways they rank higher than Sycamore in
the pantheon of most favoured trees by saproxylic ecologists (I recall the
immense outcry amongst entomologists when it was proposed to fell the ancient
Limes of Queen Anne's Drive at Windsor Great Park).

Keith Alexander recording the saproxylic features of an ancient Lime at Burghley Park in March 2016

Close-up details of sap run and young fruiting bodies of Ganoderma on the same ancient Lime

We thenmove to Horse Chestnut, a southern European
species that is so favoured as a parkland species. Dense foliage and tough
leaves give the impression that this is not a species to be loved by the
ecologist. BUT, like Sycamore, it develops fantastic rot holes and sap runs.
Its decay processes lead to excellent subterranean decaying timber, and it
grows quickly; thereby creating new habitat for some species where other
longer-lived trees may fail to deliver in time.

And, finally, what about Sweet
Chestnut? Now this is one that I have never been fond of. But, I am a
Dipterist. Were I to be a Coleopterist, I might think differently, as Sweet Chestnut
supports many of the important heart rot fungi of Oak, and as such it also
harbours many of the noteworthy beetles that like dry red heart rot.

Thus, I hear the calls - why are you
an apologist for the 'Foreign Invader'? Well, I am not! But, in today's World I
think we have to start to think in broader terms. We have already lost the Elms
that supported many important Diptera. Thank goodness for Sycamore and Horse
Chestnut that have maintained the necessary habitat. Various Oak diseases
threaten our iconic ancient trees and thus the beetles and other invertebrates
that utilise decaying timber. Thank goodness for Sweet Chestnut!

And then there is the stately hybrid
lime! A magnificent feature tree, fast growing and capable of supporting a wide
range of saproxylic invertebrates.

Now, I am not advocating replanting
the landscape with non-natives; but I think in the current climate where new
pathogens threaten out native trees such as Ash, we have got to look seriously
at the possible alternatives that will allow our native fauna to survive!

Thursday, 10 March 2016

From time to time the question
arises as to the merits of particular trapping techniques. Different people
hold differing views. Everything depends upon what you want to achieve and how
deep ones pockets are both in terms of time and capacity to get material
identified. I sometimes come across projects in which a Malaise trap has been employed without real thought being given the the scale of the undertaking, so perhaps a few notes may be helpful for those readers who are thinking about invertebrate surveys.

If one wants to develop a full
list of species from a site,a range of trapping
techniques may be necessary. It may also help to employ a number of different specialists to undertake
active sampling: each insect Order requires detailed
knowledge of how to find them that comes from specialisation. The techniques used by Dipterists differ from
Coleopterists for example - Dipterists generally swish gently through
vegetation using a light net, whereas Coleopterists use much heavier equipment
and tend to thrash the vegetation. Engaging large numbers of specialists can be hugely expensive and is probably not justifiable in any but
the most extreme circumstances.

Targeted surveys are far more
likely to be required. Knowing something about the target organisms is
essential before one employs a particular technique or combination of
techniques. Understanding something about the ecology of a site is also really important: there is perhaps less point in employing a Dipterist in oak-dominated wood pasture, whereas specialist dipterological survey is essential on base-rich flushes. In other words, think about outcomes rather than simply collect data and then wonder what you can do with it!

If quantitative data are required, then sampling that must be repeatable by others. In the 1980s the then Nature Conservancy Council
had three survey teams (Welsh Peatlands, East Anglian Fens and Dungeness) who
all used a combination of pitfall and water traps. I ran the Dungeness survey,
which was highly illuminating! If nothing else, it highlighted the huge commitment of time that was needed to set and service traps; and the even larger time commitment needed to sort the sample and get it identified.

Pitfall/Water trap combinations

The effort required to set out
and harvest the samples involved two to three
days (by two people) on each occasion for about 40 sample locations.

There was a huge volume of
specimens, and although we sorted to family for many Orders, it proved
impossible to get all of the samples identified. At the time, we used a
combination of paid and voluntary effort. I doubt many would volunteer to do
such identification today!

Water traps were very effective
in sampling aculeate Hymenoptera and I am convinced they are really useful for
sampling spider-hunting wasps. I am less convinced of their efficacy for hoverflies:
one generally gets a sample of a few common species, but not much else. I also
identified the hoverflies from the water traps run in East Anglia; these were
rather more interesting and did yield incredible numbers of Neoascia on some occasions. They were
also quite useful for Sciomyzidae (as were some pitfall traps).

Pitfall traps sometimes generated huge catches of a single species - many hundreds of isopods, anything up to several hundreds of the ground beetle Calathus fuscipes and large numbers of ants if placed close to a colony. The overall species list for a set of traps was often substantial but it had very few dominant species and a long tail of occasional records that made multivariate analysis unconvincing. Cruder analysis showed the relative frequency of some
interesting animals across the sites, but making a match to the NVC communities
proved to be almost impossible. The best splits occurred where there was a big
difference in environmental parameters; whereas different lichen heath
communities showed very little difference in the assemblage.

Perhaps with more analytical time
we might have made something of the data but this was not available to us. It
illustrates the need to build in a lot of time for analysis!

What about the Malaise trap?

Correctly sited, Malaise traps
can generate phenomenal numbers of specimens across a wide range of taxa. But,
the critical issue is getting the orientation right. It is important to
remember that Malaise traps will intercept tourist species, so one
must treat species lists based on Malaise traps with some caution. Not
everything will be resident on the site.

Obviously, a single Malaise trap
is not sufficient to generate data that can be used for multivariate analysis.
Many traps are needed, and this means huge effort and vast volumes of by-catch.
This would generally rule out Malaise traps as a practical way of sampling for
detailed ecological studies. Nevertheless, a single Malaise trap run over a
number of years may help to generate valuable data for trend analysis. This was
very much the case for Jenny Owen's study of her Leicestershire garden.

The real challenge is what to do
with trapped material? There are relatively few specialists who will take huge
volumes of specimens for free. So, anyone thinking of running a Malaise trap
needs to think very carefully about how the sample will be identified. If an
animal has died then the least one owes it is to get a name and to create a
useful record!

In general I am not a great fan
of Malaise traps unless they are used for a specific purpose and there is the
capacity to deal with the catch. Using them to live-catch may be more helpful
where it is possible to regularly monitor the collecting head and to release
live unwanted specimens.

So what are the other
alternatives?

The main point to consider is
that one needs to think out carefully precisely what is wanted:

an inventory of species for a site?

sufficient data to undertake multivariate
analysis?

long-term monitoring?

autecology of a single species or suite of species associated with a highly defined habitat?

Sweep net sampling is often used
(e.g. Natural England site condition monitoring), with the quantitative sample
generated by using a given number of sweeps. For one-off studies, this may be
enough to generate useful and repeatable data. Unfortunately, no two net operators will generate the same
sample and so year-on-year data will not be comparable if different
entomologists are used to sample the site.

Active searching may also be
helpful. Coleopterists often
target a range of habitats such as under objects or within decaying wood. Some
such systems are rather destructive and are perhaps not to be recommended in
sensitive locations. In the case of hoverflies we might seek out larvae such as those of leaf and stem-miners, or perhaps those found in rot holes.

For hoverflies, I generally
favour active searching and sweeping. My form of active searching is to make
detailed observations of suitable sunning and feeding stations as well as
investigating possible breeding sites. I might use a Malaise trap under
specific circumstances but would generally avoid water traps (they are my
favoured system for sampling Pompilidae though).

The crucial point about all of the options is to design the project in consultation with an experienced entomologist who has some understanding of the logistics. Invertebrate recording is complex, time-consuming and expensive, and relies upon a very small nucleus of experienced entomologists. A few hours of such a person's time at the start of a project can save an awful lot of money (and save the lives of many insects).

Wednesday, 9 March 2016

This last weekend I was asked which type of recording I favoured? A
patch or wide-ranging recording? Bearing in mind a recording scheme is often
judged by geographic coverage, it may come as a surprise to some that I
thoroughly endorsed the 'patch' approach. Why might this be? Well, the big
advantage of the 'patch' and regular recording, is that one starts to build up
a year-on-year picture of a the local environment, with sufficient data to
start to pick up trends. That is what we have lacked and what is needed to help
to build the contextual picture.

Some context

Before the days of cheap transport, field naturalists tended to have a
'patch' and took an interest in many aspects of the natural history of their
chosen site. Unfortunately, data collection systems were confined to card
indexes and notebooks; many of which have probably been lost. The lack of
systems to capture data mean that on the whole we have only a sparse record of
what occurred prior to the 1960s. For invertebrates, museum collections are a
great source of information, but inevitably they tend to hold examples of rarer
species: there is not the space to retain all specimens of commoner species,
recorders also limited the extent of their collections. Doubtless there were
those who placed a lot of emphasis on seeing rarer species, but the 'patch'
was an important part of their biological recording activity.

Cheap transport has allowed many naturalists to travel much further
afield in pursuit of their chosen interest. In Entomology it is commonplace for
field meetings to be organised at the richest sites or places where rarities
might be found. I was no different to others in the early 1980s when one would
turn up at a given locality for X or Y and find that others had arrived with
the same intentions. Our collections were very similar and this took mothing
into the realms of stamp collecting; hardly an ideal foundation for biological
recording . I suppose that in a way we were the forerunners of today's moth
twitchers who visit other moth traps to see the poor little soul that has flown
off course, ended up in a moth trap and then in a pot in a fridge. What does
this contribute to our knowledge of species' biology and conservation status?
Precious little!

Thank goodness there was the Rothampstead Insect Survey, which
established a network of locations where all moths were recorded. Although
greatly trimmed in its coverage, this dataset is perhaps THE most important for
invertebrate conservation. Its findings convey a sad story of decline in
Britain's moth fauna. Efforts by the National Moth Recording Scheme (NMRS) to
assemble the data from individual moth traps compliment the Rothampstead
survey, but will always lack the benefits of the consistent approach that the
Rothampstead survey has established. Some NMRS records will be from
mobile traps that are run in more interesting places, but many will be people's
gardens - the equivalent of the 'patch'.

Data for butterfly transects are also pretty robust and it is possible
to say quite a lot about what has happened to them over the past 40 years. Lots
of organisations and individuals run butterfly transects and these provide a
hugely important resource that helps to chart the fortunes of Britain's
butterflies.

Long-term monitoring of other taxa fares less well. There are
comparatively fewer enthusiasts, and although there are recording schemes, the
volumes of data for more challenging taxa are relatively sparse. This is
arguably a serious gap in our knowledge because we cannot automatically
extrapolate from the fortunes of moths and butterflies. The Lepidoptera occupy
a relatively narrow niche: the vast majority are phytophages, whereas other
Orders occupy much wider ranges of niches. Guilds of saprophages, mycophages,
filter-feeders, parasites and predators are represented across the insect World
beyond the Lepidoptera, and although we do have data they are nowhere near
as robust. Nevertheless, it is still possible to detect trends; many of
which mirror those for the Lepidoptera - perhaps 40-50% declining, 30-40%
stable and 10-20% increasing.

For hoverflies we now have a run of about 35 years data in which more
than 20,000 records have been submitted each year. This dataset is hugely
important because hoverflies occupy a very broad range of niches (apart from
parasitism) and can be used to convey important stories about the state of the
wider countryside. The dataset is one of the bigger invertebrate datasets and
it is possible to extract trends from the data for some but not all species
(most notably lacking are the rarest species). The results are not pretty,
paralleling those for many other taxa with about 40% of species declining
significantly. Unfortunately, we don't have the long runs of data for
individual sites like those of the butterfly or moth monitoring schemes.

How do we make something of the
data?

There are various analytical techniques that can be used to examine the
degree to which data are fully representative of a given area. Several work in
similar ways, comparing the presence of given species against a basket of
commoner species to determine the intensity of recording, and thus the
likelihood that a species' absence is a function of recorder effort rather than
being an actual absence. The system develops a 'matrix of confusion' which is
somewhat Rumsfeldian in its outputs:

Locations where the species is known
to occur and the model predicts that it will occur;

Locations where the species has not
been recorded and the model predicts that it will not be found;

Locations where the species is known
to occur but it is not predicted by the model; and

Locations where the species has not
been found but the model predicts its occurrence.

The 'matrix of confusion is used to test the reliability of predictive
models and is highly dependent upon the availability of records for both
commoner and rarer species. A measure of recorder effort can be determined from
the proportion of commoner species that might be expected to occur that have
actually been recorded in a given square and in surrounding squares.

Where species X is present in a given square and say only 40% of the
commoner species that are likely to be constants in full species lists are present,
this represents a very positive result. If it is absent, there remains the
possibility that it will occur, assuming the location has the appropriate
environmental parameters. Conversely if species X is absent from a square where
all 50 of the commonest species have been recorded, it may be inferred that the
level of recording has probably been sufficient to generate a reasonably
comprehensive picture of the fauna of that square.

If only rarer species are recorded, the analytical outcome is likely to
be a little distorted. Conversely, if species X is challenging to identify, it
is highly likely to be under-represented in data that is dominated by records
of easily identified species. The more comprehensive the dataset, the better
the outcome.

Making something of trends on ad-hoc data is always going to be more
difficult than an analysis of data collected using a constant system of survey
such as butterfly transects, water traps, pitfall traps or a Malaise trap. But,
regular repeat recording from a given 'patch' can result in remarkably valuable
data. For example, Alan Stubbs has undertaken regular monitoring of his
Peterborough garden. In the 25 years or so that he has done this, it has shown
that overall species richness has dropped quite substantially. Stuart Ball uses
Alan's data and the data generated by Jenny Owen's Malaise trap in Leicester as
the only really detailed data that help to provide a picture of environmental
change for hoverflies. Jenny stopped running her trap some while ago (and died
recently), so that data run has come to an end. Alan is still monitoring his
garden, but of course it is just one dataset. What we need is more people who
are willing to regularly record from their garden or a favoured patch.

Signs of progress

In the last year, several members of the UK Hoverflies Facebook page
have been very active, regularly recording from their 'patch'. This is a
welcome boost to the data and if it continues it will start to form the
foundations for a pretty decent long-term monitoring project. It parallels the
general concept of the garden hoverfly monitoring scheme that we have been
trying to develop. Obviously, some people will drop out in time, but others may
be recruited. The development of a big dataset from several constant effort
sites should provide a really good foundation for the future.

So, for those who like to have a 'patch' do please make the effort to
regularly record the hovers. Those who favour more wide-ranging activities -
yes please, and don't forget the full species list. The critical issue is the
full list of what is seen and can be reliably identified. Hopefully we can
develop a system like Birdtrack, which is a really neat way of using 'ad hoc'
data to demonstrate aspects of species' biology and to investigate trends at a
national or regional level. And, if you do want to twitch special places, do
please make sure the records reach the scheme - it is amazing how few records
we see for New Forest specialities such as Caliprobola speciosa!

About Me

An ecologist with over 30 years experience in statutory nature conservation and biological recording. I am now an independent coastal management consultant (Bright Angel Coastal Consultants Ltd.) and have undertaken commissions in the UK and several northern European countries. I originally trained as an entomologist and parasitologist. Today, I retain an interest in entomology but in a strictly non-vocational capacity. In my non-working time I am joint organiser of the Hoverfly Recording Scheme (with Dr Stuart Ball) and am extremely committed to developing new capacity in insect identification and biological recording.